to Elcctriciiij and Magnetism. 493 



an iniportfint part of my paper, and is intimately connected 

 with almost all the pha^nomena subsequently described in the 

 communication, I was, of course, anxious to discover the cause 

 of so remarkable a discrepancy. There could be no doubt 

 of the truth of my results, since a shock from a secondary 

 current which would paralyze the arms was so much reduced 

 by the interposition of plates of metal, as scarcely to be felt 

 through the tongue. 



3-t. After some reflection, however, the thought occurred 

 to me that induction might be produced in such a way as not 

 to be affected by the interposition of a plate of metal. To 

 understand this, suppose the end of a magnetic bar placed 

 perpendicularly under the middle of a plate of copper, and a 

 iielix suddenly brought down on this; an induced current 

 would be produced in the helix by its motion towards the 

 plate, since the copper, in this case, could not sci'een the 

 magnetic influence. Now, if we substitute for the magnet a 

 coil through which a galvanic current is passing, the effect 

 should be the same. The experiment was tried by attaching 

 the ends of the helix to a galvanometer*, and the result was 

 as I expected : when the coil was suddenly brought down 

 on the plate the needle swung in one direction, and when 

 lifted up, in the other; the amount of deflection being the same, 

 whether the plate was interposed or not. 



35. It must be observed in this experiment, that the plate 

 was at rest, and consequently did not partake of the induction 

 produced by the motion of the helix. From my previous in- 

 vestigations, I was led to conclude that a different result 

 would follow, were a current also generated in the plate by 

 simultaneously moving it up and down with the helix. This 

 conclusion, however, was not correct, for on making the ex- 

 periment, I found that the needle was just as much affected 

 when the plate was put in motion with the helix as when the 

 latter alone was moved. 



36. This result was so unexpected and remarkable, that it 

 was considered necessary to repeat and vary the experiment 

 in several ways. First, a coil was interposed instead of the 

 plate, but whether the coil was at rest or in motion with the 

 helix, with its ends separated or joined, the effect on the 

 galvanometer was still the same; not the least screening 

 influence could be observed. In reference to the use of 

 the coil in this experiment, it will be recollected that I have 



* Tlie arrangerncnt will be readily iiiidcrstood by su[)posiiig in Fig. 3. 

 the Iiundles removed, and tlic ends of the iielix joined to the ends of the 

 wire of a galvanometer; also, by a plate of metal interposed between the 

 helix and the coil. 



